California utility Pacific Gas and Electric Company said Thursday it has entered into an agreement to buy power from a hybrid project that will combine solar-thermal and biofuel technology to make electricity 24 hours a day.

PG&E signed the contract with San Joaquin Solar, a renewable energy unit of Portuguese holding group Martifer. San Joaquin will build two identical hybrid plants, totaling 106.8 megawatts, in central California near the town of Coalinga, Calif. The plants are expected to deliver a total of 700 gigawatt hours of electricity annually to PG&E customers in northern and central California.

In the past year, PG&E and other utilities have signed a series of contracts with other companies to help bring more solar-thermal electricity to the grid.

Earlier this month eSolar, financed by Google’s investment arm, signed a deal to build 245 megawatts of concentrating solar-thermal plants for Southern California Edison (see California to Get More Solar-Thermal).

Among other examples, BrightSource Energy in April announced it will provide up to 900 megawatts of solar-thermal power to PG&E; Ausra in November announced a 177-megawatt agreement with the utility; and in July last year PG&E announced an agreement to buy 553 megawatts of electricity from a solar power plant to be built by Israeli company Solel (see Ausra to Build 177-Megawatt Solar-Thermal Plant, Ausra Raises $40M for Concentrating Solar-Thermal and FPL and PG&E Back Solar-Thermal).

Solar-thermal power plants use the sun's heat instead of its light to produce electricity. Companies use different methods to harvest the heat, including the use of mirrors to direct sunrays toward a water tower. As the water heats up, it turns into steam, which can then be converted into electricity using a turbine.

San Joaquin will use solar-thermal technology developed by Luz International.

San Joaquin’s project offers something different than other PG&E solar thermal plants. The new plant will include a system that uses biomass to power the turbines in order to keep them going after the sun sets, said Jennifer Zerwer, a PG&E spokesperson.

PG&E and San Joaquin tout this type of hybrid project as the first for the utility. However, the biomass component of the project is using technology that can easily be adopted. The challenge will be to assemble the pieces and engineer them to work well together.

The project will use agricultural and residential green waste, such as tree branches and grass, and burn it to boil water. Steam from the water will then power the turbine.

Each hybrid project will need 250,000 tons of biomass annually, which is why the projects are located in a farming community and next to a major interstate highway, said Andrew Byrnes, a spokesperson for Spinnaker Energy, which will take part in the project.

"Everyone has to truck their waste somewhere," he said.

Byrnes wouldn't say how much the projects would cost. However, he said the hybrid approach would help produce more energy for the money when compared to standalone solar thermal project.

The projects are expected to start producing energy in 2011.

So what's to stop other solar-thermal companies from tacking on a biomass-powering component to help produce electricity at night? One problem could be the distance between the plant and an abundant feedstock.

According to Zerwer, BrightSource and Solel are building their plants in the Mojave Desert.